Method and apparatus for distortion correction in optical communication links

ABSTRACT

In some embodiments, an apparatus includes an optical transmitter module that can be electrically coupled to an electrical serializer/deserializer and a controller. The optical transmitter module can include an electrical detector that can receive an in-band signal. The electrical detector can send to the controller a first power error signal and a second power error signal based on the in-band signal. The controller can send a correction control signal to the electrical serializer/deserializer based on the first power error signal and the second power error signal such that the electrical serializer/deserializer sends a pre-emphasized signal to the optical transmitter module based on the correction control signal. In such embodiments, the first power error signal, the second power signal and the correction control signal are out-of-band signals.

BACKGROUND

Some embodiments described herein relate generally to methods andapparatus for the detection and correction of distortion of thetransmitted signal in an optical transmitter. In particular, but not byway of limitation, some of the embodiments described herein relate tomethods and apparatus for the detection and correction offrequency-based distortion of the transmitted signal associated withboth an analog implementation and a digital implementation of an opticaltransmitter.

High data rate optical networks (e.g., 100 Gbit/s and beyond) can beenabled by, for example, an optical M-ary quadrature amplitudemodulation (M-QAM) scheme with digital signal processing (DSP).Transmitter side serializer/deserializer (SerDes) and digital-to-analogconverters (DACs) are building blocks for spectrally-efficient,multi-level signal generation and spectral manipulation. High data ratecommunication interfaces, however, can impose high signal integritydemands that are difficult to accomplish without the use of pre-emphasisand/or post-compensation techniques. Known methods of pre-emphasis basedsignal correction include providing variable analog peaking in a SerDesalong the transmitter-side orientation (TX) or the use of a digitalfilter in conjunction with a DAC.

Adjusting the pre-emphasis parameters of the SerDes TX can beparticularly difficult if the interconnect properties are not known atthe time of manufacture and the pre-emphasis parameters cannot be setaccurately in the factory. This is typically the case when pluggablephotonic elements are used, where the properties of the photonicelements can vary dramatically from vendor to vendor and overgenerations of photonic elements. Hence, this presents challenges forcomponent designers and board designers that can prevent in-factorycalibration and the correction of distortion.

Accordingly, a need exists for methods and apparatus for automaticallysetting the SerDes TX pre-emphasis parameters for a specific opticaltransmitter system. Such optimization can be implemented at the time ofport turn up, after reset, or continuously optimized in the backgroundto compensate for signal power fluctuations due to, for example,temperature effects and/or temporal effects.

SUMMARY

In some embodiments, an apparatus includes an optical transmitter modulethat can be electrically coupled to an electricalserializer/deserializer and a controller. The optical transmitter modulecan include an electrical detector that can receive an in-band signal.The electrical detector can send to the controller a first power errorsignal and a second power error signal based on the in-band signal. Thecontroller can send a correction control signal to the electricalserializer/deserializer based on the first power error signal and thesecond power error signal such that the electricalserializer/deserializer sends a pre-emphasized signal to the opticaltransmitter module based on the correction control signal. In suchembodiments, the first power error signal, the second power signal andthe correction control signal are out-of-band signals.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a system block diagram of an optical transmitter system with acontrol module, according to an embodiment.

FIG. 2 is a system block diagram of an optical transmitter system with acontrol module, according to an embodiment.

FIG. 3 is a system block diagram of a control module, according to anembodiment.

FIG. 4 is a graphical example of an uncompensated signal (or channel), apre-emphasis (transfer or adjustment) function and the resultingcompensated (or equalized) signal (or channel).

FIG. 5 is a graphical example of time domain eye diagrams for: (a) anuncompensated broadband on-off-keyed signal; and (b) a compensatedbroadband on-off-keyed signal.

FIG. 6 is a flow chart illustrating a method for the compensation offrequency-based transmitted signal power fluctuations, according to afirst embodiment.

FIG. 7 is a flow chart illustrating a method for the compensation offrequency-based transmitted signal power fluctuations, according to asecond embodiment.

DETAILED DESCRIPTION

In some embodiments, an apparatus includes an optical transmitter modulethat can be electrically coupled to an electricalserializer/deserializer and a controller. The optical transmitter modulecan include an electrical detector that can receive an in-band signal.The electrical detector can send to the controller a first power errorsignal and a second power error signal based on the in-band signal. Thecontroller can send a correction control signal to the electricalserializer/deserializer based on the first power error signal and thesecond power error signal such that the electricalserializer/deserializer modifies a pre-emphasis function, based on thecorrection control signal, and applies the pre-emphasis function toincoming signals to generate and send a pre-emphasized signal to theoptical transmitter module. In such embodiments, the pre-emphasisfunction can be a transfer function or an adjustment functions and thefirst power error signal, the second power signal and the correctioncontrol signal can be out-of-band signals.

In some embodiments, an apparatus includes a controller that isoperatively coupled to an electrical serializer/deserializer and anoptical transmitter module having an electrical detector. The controllercan receive from the electrical detector a first power error signal anda second power error signal based on an in-band signal. The controllercan send a correction control signal to the electricalserializer/deserializer based on the first power error signal and thesecond power error signal such that the electricalserializer/deserializer sends a pre-emphasized signal to the opticaltransmitter module based on the correction control signal. In suchembodiments, the first power error signal, the second power signal andthe correction control signal are out-of-band signals.

In some embodiments, an apparatus includes an electricalserializer/deserializer that is operatively coupled to a controller andan optical transmitter module having an electrical detector. Theelectrical serializer/deserializer can send to the electrical detectoran in-band signal such that the optical transmitter module sends to thecontroller a first power error signal and a second power error signal.The electrical serializer/deserializer can receive from the controller acorrection control signal based on the first power error signal and thesecond power error signal, and can send to the optical transmittermodule a pre-emphasized signal based on the correction control signal.In such embodiments, the first power error signal, the second powersignal and the correction control signal are out-of-band signals.

As used in this specification, the terms “controller” and “controlmodule” are used interchangeably, unless the context clearly dictatesotherwise, and can refer to any hardware and/or software module that canreceive a first power error signal and a second power error signal andgenerate a correction control signal based on the first power errorsignal and the second power error signal.

As used in this specification, the terms “electricalserializer/deserializer” and “serializer/deserializer module” are usedinterchangeably, unless the context clearly dictates otherwise, and canrefer to any hardware and/or software module that can receive acorrection control signal and generate and/or modify a pre-emphasizedsignal based on the correction control signal.

As used herein, a module can be, for example, any assembly and/or set ofoperatively-coupled electrical components associated with performing aspecific function, and can include, for example, a memory, a processor,electrical traces, optical connectors, software (stored in memory and/orexecuting in hardware) and/or the like.

As used in this specification, the singular forms “a,” “an” and “the”include plural referents unless the context clearly dictates otherwise.Thus, for example, the term “an optical transmitter system” is intendedto mean a single optical transmitter system or multiple opticaltransmitter systems.

FIG. 1 is a system block diagram of an optical transmitter system with acontrol module, according to an embodiment. The optical transmittersystem 100 includes an optical transmitter module 110 and aserializer/deserializer module 130 that are operably coupled to eachother. The optical transmitter module 110 and theserializer/deserializer module 130 are both operably coupled to acontrol module 120. Although the control module 120 is shown in FIG. 1as being external to the optical transmitter system 100, in otherconfigurations, the control module 120 can be internal to the (i.e., apart of) optical transmitter system 100. The optical transmitter system100 can be any high data rate optical transmitter system such as, forexample, an optical M-ary quadrature amplitude modulation (M-QAM)transmitter, a polarization multiplexed (PM) M-QAM transmitter, and/orthe like. The optical transmitter module 110 can include a set ofMach-Zehnder interferometers (or modulators) for the in-phase portion(I-channel or I-phase modulator) and the quadrature portion (Q-channelor Q-phase modulator), respectively, of the optical transmitter system100. The I-phase modulator can be coupled to a first electrical detector(not shown in FIG. 1) and the Q-phase modulator can be coupled to asecond electrical detector (not shown in FIG. 1). Additionally, theoptical transmitter module 110 can include, for example, a tunable lasersource, an optical shutter (for preventing the calibration signals frombeing transmitted and/or corrupting the transmitted signal as explainedin greater detail herein), one or multiple amplifiers (or drivers), andone or multiple electrical detectors. The different components of theoptical transmitter module 110 will be discussed in greater detail inFIG. 2.

As shown in FIG. 1, the optical transmitter module 110 can beelectrically coupled to the serializer/deserializer module 130 and thecontrol module 120. The optical transmitter module 110 can include anelectrical detector (not shown in FIG. 1) that can receive an in-bandsignal from the serializer/deserializer module 130. In some instances,the optical transmitter module 110 (i.e., the electrical detector) canfilter the in-band signal at a first frequency range to produce a firstfiltered signal, and can filter the in-band signal at a second frequencyrange different from the first frequency range to produce a secondfiltered signal. In such instances, the optical transmitter module 110can generate a first power error signal associated with the firstfiltered signal and a second power error signal associated with thesecond filtered signal. In such instances, the optical transmittermodule 110 can send to the control module 120, the first power errorsignal and the second power error signal based on the in-band signal.

In other instances, the optical transmitter module 110 can receive fromthe serializer/deserializer module 130, a first calibration signal(in-band signal) having a pre-determined digital bit stream associatedwith a first frequency, and a second calibration signal (in-band signal)having a pre-determined digital bit stream associated with a secondfrequency (that is different from the pre-determined digital bit streamassociated with the first frequency). In such instances, the opticaltransmitter module 110 can generate a first power error signal that isassociated with the first calibration signal, and a second power errorsignal is associated with the second calibration signal. In suchinstances, the optical transmitter module 110 can send to the controlmodule 120, the first power error signal based on the first calibrationsignal and the second power error signal based on the second calibrationsignal.

The control module 120, in some configurations, can be a stand-alonehardware module that can be external to the optical transmitter system100 as shown in FIG. 1. In other configurations, the control module 120can be a hardware module located on the host circuit board of theoptical transmitter system 100. In yet other configurations, the controlmodule 120 can be software module stored in the memory and/or executedin the processor of the optical transmitter system 100. The controlmodule 120 can be operatively coupled to the electricalserializer/deserializer module 130 and the optical transmitter module110 (that includes one or multiple electrical detectors). The controlmodule 120 can receive the output of the electrical detectors in theoptical transmitter module 110 that can include the first power errorsignal and the second power error signal (that are either based on anin-band signal, or a first calibration signal and a second calibrationsignal). The control module 120 can compute the parameters orcharacteristics of a correction control signal based on the propertiesor characteristics of the first power error signal and the second powererror signal (e.g., magnitude of the signals, phase of the signals,frequency of the signals, etc.). The control module 120 can generate acorrection control signal based on parameters or characteristicscomputed and can send the correction control signal to the electricalserializer/deserializer module 130 such that the serializer/deserializermodule 130 can generate and/or modify a pre-emphasized signal to theoptical transmitter module 110 based on the correction control signal.

The serializer/deserializer module 130 can be a hardware module of theoptical transmitter system 100. The serializer/deserializer module 130is operably coupled to the optical transmitter module 110 (that includesone or multiple electrical detectors) and the control module 120 as seenin FIG. 1. The serializer/deserializer module 130 can include, forexample, a serializer-deserializer/digital-to-analog converter(SerDes/DACs) module, a calibration signal generation module and apre-emphasis generation module (not shown in FIG. 1, but described ingreater detail in FIG. 2). In some instances, theserializer/deserializer module 130 can send to the electrical detectorof the optical transmitter module 110 an in-band signal, such that theoptical transmitter module 110 can send to the control module 120 afirst power error signal and a second power error signal. In otherinstances, the serializer/deserializer module 130 can send to theoptical transmitter module 110 a first calibration signal having apre-determined digital bit stream associated with a first frequency anda second calibration signal having a pre-determined digital bit streamassociated with a second frequency such that the optical transmittermodule 110 can send to the control module 120 a first power error signaland a second power error signal. The serializer/deserializer module 130can receive from the control module 120 a correction control signal(that is based on the first power error signal and the second powererror signal). In some configurations, the serializer/deserializermodule 130 can generate or produce a pre-emphasized signal (based on thecorrection control signal) that compensates for or substantiallycompensates for the frequency-dependent distortions in the opticaltransmitter system 100. As discussed above, in some configurations, theserializer/deserializer module 130 can include aserializer-deserializer/digital-to-analog converter (SerDes/DACs)module. In some instances, the (SerDes/DACs) module can include a firstdigital-analog converter (DAC) and a second DAC. In such instances, theserializer/deserializer module 130 can send a first calibration signalfrom the first DAC to a first electrical detector; theserializer/deserializer module 130 can also send a second calibrationsignal from the second DAC to a second electrical detector. In suchinstances, the I-phase modulator is coupled to the first electricaldetector and the Q-phase modulator is coupled to the second electricaldetector.

FIG. 2 is a system block diagram of an optical transmitter system with acontrol module, according to an embodiment. The optical transmittersystem 200 includes an optical transmitter module 210 that is operablycoupled to a serializer/deserializer module 280. The optical transmittermodule 210 and the serializer/deserializer module 280 are also operablycoupled to a control module 260 that is external to the opticaltransmitter system 200. Although the control module 260 is shown in FIG.2 as being external to the optical transmitter system 200, in otherconfigurations, the control module 260 can be internal to (part of) theoptical transmitter system 200.

The optical transmitter module 210 can be a hardware module in theoptical transmitter system 200 and can include a tunable laser 212, amodulator structure 215 that includes an I-channel modulator 216 and aQ-channel modulator 218, a (optical) power monitor 219, an opticalshutter 220, amplifiers (or drivers) 222 and 224, and (electrical)detectors 230 and 234. The serializer/deserializer module 280 includes aSerDes/DAC module 282, a pre-emphasis generation module 284, and acalibration signal generation module 286. The output of the detectors230 and 234 are connected to the control module 260 and the output ofthe control module 260 is connected to the pre-emphasis generationmodule 284 and the calibration signal generation module 286.

The tunable laser 212 can provide an electromagnetic continuous wave ata carrier frequency on which information can be imposed by the opticaltransmitter system 200 by, for example, increasing the carrier signalstrength, varying the carrier base frequency, varying the carrier wavephase, or by other means to transmit signals (or data) to externaldevices. The output from the optical transmitter system 200 is themodulated and transmitted signal. The modulator structure 215 canreceive an incident optical signal (incident light) from the tunablelaser 212 and can split the incident optical signal into a first opticalsignal and a second optical signal. The first optical signal can be sentto the I-channel modulator 216 and the second optical signal can be sentto the Q-channel modulator 218. The optical signals from the I-channelmodulator 216 and the Q-channel modulator 218 can be interferometricallycombined to form a single optical signal with information imparted inboth the magnitude and phase of the optical signal. Note in someinstances, the output optical signal from the modulator structure 215can be combined with the optical output signal from a second modulatorstructure (not shown) such that the polarization of the optical outputsignal from the first modulator structure 215 is substantiallyorthogonal to that from the second modulator structure once combined.

As mentioned above, the modulator structure 215 includes an I-channelmodulator 216, and a Q-channel modulator 218. The I-channel modulator216 can be, for example, a Mach-Zehnder modulator (MZM) associated withthe in-phase portion of the optical transmitter system 200. Similarly,the Q-channel modulator 218 can also be, for example, a MZM associatedwith the quadrature-phase portion of the optical transmitter system 200.The combined output signal of the modulator structure 215 can bedetected or monitored by the (optical) power monitor 219. The opticalpower monitor 219 can be, for example, any type of low-speed,low-bandwidth optical detectors available commercially that can be usedfor the detection of the combined output of the modulator structure 215.The optical shutter 220 can have a first configuration and a secondconfiguration. In some instances, when calibration signals(pre-determined digital bit streams) are sent from theserializer/deserializer module 280 to the optical transmitter module210, the optical shutter 220 can be in the first configuration (closed)and can block output from the optical transmitter system 200. In otherinstances, when no calibration signals are sent from theserializer/deserializer module 280 to the optical transmitter module210, the optical shutter 205 can be in the second configuration (open)and can transmit output from the optical transmitter system 200.

The detectors 230 and 234 can be electrical detectors that can be used,in some instances, to measure the power of the calibration signals sentfrom the serializer/deserializer module 280 to generate a first powererror signal and a second power error signal. In other instances, thedetectors 230 and 234 can be electrical detectors that can implement afiltering functionality to filter an in-band input signal sent from theserializer/deserializer module 280 at two different frequency ranges togenerate a first filtered signal and a second filtered signal. In suchinstances, the detectors 230 and 234 can also measure the power of thefirst filtered signal and the second filtered signal to generate a firstpower error signal and a second power error signal. For example, thedetectors 230 and 234 each can be, for example, a root mean square (RMS)detector that outputs a direct current (DC) voltage that is linearlyproportional to the log of the input signal power, a threshold detectorthat uses an external resistor or threshold voltage and can output atransistor-transistor logic (TTL) compatible signal when the inputsignal power level exceeds the preset threshold, a log power detectorthat can provide a DC output voltage that is log-linearly proportionalto the input signal power level, an Schottky peak detector that cancombine a temperature compensated Schottky diode peak detector and abuffer amplifier to detect the input signal voltage peak using theon-chip Schottky diode, and/or the like. After measuring or detectingthe power of the calibration signals or the filtered signals, thedetectors 230 and 234 can send the first power error signal and thesecond power error signal (that includes the output voltage levelrepresentative of the power of the calibration signals or the filteredsignals) to the control module 260 as shown in FIG. 2.

The (analog) output signals from the SerDes/DAC module 282 are sent tothe amplifiers (or drivers) 222 and 224 that are associated with theI-channel and the Q-channel of the optical transmitter system 200,respectively. The amplifiers 222 and 224 can be a type of electronicamplifier that can be implemented in an integrated circuit on a chip(hardware) that can convert a low-power input signal into a higherpowered signal for, for example, driving a high powered device such asthe I-channel modulator 216 and/or the Q-channel modulator 218, and/orthe like. The amplifiers 222 and 224 can be optimized, for example, tohave high efficiency, high output power compression, low return loss,high gain, and optimum heat dissipation. The amplified (analog) signalsfrom the amplifiers 222 and 224 are sent to the I-channel modulator 216and the Q-channel modulator 218, respectively, of the opticaltransmitter system 200.

The control module 260 can be a hardware module and/or software modulestored in the memory and/or executed in a processor of a stand-alonedevice that, in some configurations, is external to the opticaltransmitter system 200. In other configurations, the control module 260can be a hardware module and/or software module stored in the memoryand/or executed in a processor located on the host circuit board of theoptical transmitter system 200. The control module 260 receives theoutput of the detectors 230 and 234 (of the optical transmitter module210) that can include the first power error signal and the second powererror signal and are representative of the input signal powerfluctuations due to, for example, temperature changes and/or temporaleffects. The control module 260 can generate a correction control signalthat is based on the first power error signal and the second power errorsignal. The control module 260 can send the correction control signal tothe serializer/deserializer module 280 such that theserializer/deserializer module 280 can, in some instances, generateand/or modify a pre-emphasized signal to the optical transmitter module210 based on the correction control signal. In other instances, theserializer/deserializer module 280 can generate a pre-emphasized signalbased on the correction control signal and can send the pre-emphasizedsignal to the optical transmitter module 210, where the pre-emphasizedsignal results in an equalized signal for which the frequency-dependentdistortions in the optical transmitter system 200 have been compensatedor substantially compensated. In some configurations, the control module260 can also compute the parameters of calibration signals parametersthat in some instances, can be sent from the serializer/deserializermodule 280 to the optical transmitter module 210 to detect and correctfor input signal power fluctuations due to temperature and temporaleffects. Note the calibration signals are generated by the calibrationsignal generation module 286 based on the parameters computed by thecontrol module 260. In such configurations, the control module 260 cansend a signal to the serializer/deserializer module 280 that includesinformation regarding the calibration digital bit streams that are to begenerated by the serializer/deserializer module 280.

The control module 260 can also control the bias points of the I-channelmodulator 216 and the Q-channel modulator 218, and the driving level ofthe amplifiers (or drivers) 222 and 224. The halfwave voltage, V_(pi),of a modulator (216 and/or 218) is defined as the difference between theapplied voltage at which the signals in each branch of the modulator(216 and/or 218) are in phase and the applied voltage at which thesignals are 180° out of phase. Hence, V_(pi) is the voltage differencebetween maximum and minimum output signal power of the modulator (216and/or 218). For a modulator (216 and/or 218) to be used mostefficiently in a communications system, it is desirable for the value ofV_(pi) to be accurately determined, for example, to determine theamplifier 222 and 224 settings. If the modulator (216 and/or 218) biasis set at null bias, a driving level of 2*V_(pi) from the amplifier (222and 224) can be applied. If the modulator (216 and/or 218) bias is setat quadrature bias, a driving level of V_(pi) from the amplifier (222and 224) can be applied. Additionally, the control module 260 can alsosend a signal that can control the configuration status of the opticalshutter 220 during operation of the optical transmitter system 200. Adetailed description of the functionalities of the control module 260 isprovided herein with respect to FIG. 3.

The calibration signal generation module 286 can be a hardware moduleand/or software module stored in the memory and/or executed in aprocessor of the optical transmitter system 200. In some instances, whenthe optical transmitter system 200 is in operation, the calibrationsignal generation module 286 can receive a signal from the controlmodule 260 that is representative of a first pre-determined calibrationdigital bit stream having a first frequency and a second calibrationdigital bit stream having a second frequency. The calibration signalgeneration module 286 can generate the first calibration signal and thesecond calibration signal and send the calibration signals to theamplifiers (or drivers) 222 and 224 via the SerDes/DAC module 282. Insome instances, the first calibration signal can be a firstpre-determined digital bit stream sent from the SerDes/DAC module 282 ata first frequency. In such instances, the second calibration signal canbe a second pre-determined digital bit stream with a second frequency(different from the first frequency) that is sent from the SerDes/DACmodule 282. The (digital) calibration signals can be a specific kind ofelectrical waveform varying between two voltage levels that correspondto two logic states (e.g., low state′ for ‘0’ and ‘high state’ for ‘1’,respectively). The voltage levels generated by the calibration signalgeneration module 286 can be compatible with digital electronicsinput/output (I/O) standards such as, for example, Transistor-TransistorLogic (TTL), Low-voltage TTL (LVTTL), Low Voltage Complementary MetalOxide Semiconductor (LVCMOS), Low-voltage differential signaling (LVDS),and/or the like. For example, the first calibration signal can be a“11001100” repeating pattern that can correspond to 8 GHz for a 32 Gbpsdata stream, and the second calibration signal can be a“11111111000000001111111100000000” repeating bit pattern that cancorrespond to 2 GHz for a 32 Gbps data stream.

In such instances, the power of the calibration signals can be measuredor detected by the detectors 230 and 234 to generate a first power errorsignal (based on the first calibration signal) and a second error signal(based on the second calibration signal), respectively. The detectors230 and 234 can collectively send the first power error signal and thesecond power error signal to the control module 260 as shown in FIG. 2.In such configurations, the amplifiers (or drivers) 222 and 224 aredisabled and/or the optical shutter 220 is in the closed configurationwhen the detectors 230 and 234 receive the first calibration signal andthe second calibration signal. Note that the calibration signalgeneration module 286 can generate and send the calibration signals tothe SerDes/DAC module 282 that includes theserializer-deserializer/digital-analog converters (SerDes/DACs)associated with the in-phase portion (I-channel) and quadrature portion(Q-channel), respectively, of the optical transmitter system 200. Hence,the first calibration signal and the second calibration signal are sentthrough the SerDes/DAC module 282, the amplifiers 222 and 224, and thenmodulated on an optical carrier wave via the I-channel modulator 216 andthe Q-channel modulator 218 (of the modulator structure 215),respectively.

In other instances, when the optical transmitter system 200 is inoperation, the serializer/deserializer module 280 can send an inputin-band signal to the amplifiers 222 and 224 of the optical transmittermodule 210. In such instances, the detector 230 can optionally include afilter 232 (as shown by the dashed box in FIG. 2), and the detector 234can optionally include a filter 236 (as shown by the dashed box in FIG.2). The filters 232 and 236 can filter the input in-band signal at afirst frequency range to generate a first filtered signal and a secondfrequency range (different from the first frequency range) to generate asecond filtered signal, respectively. The detectors 230 and 234 cangenerate a first power error signal based on the first filtered signaland a second power error signal based on the second filtered signal. Thedetectors 230 and 234 can collectively send the first power error signaland the second power error signal to the control module 260 as shown inFIG. 2. In such instances, the amplifiers (or drivers) 222-224 areactivated, operative or functional (i.e., not disabled) when thedetectors 230 and 234 receive the in-band input signal from theserializer/deserializer module 280. The advantage of this approach isthat the broadband input in-band signal being filtered may be actualdata, so the pre-emphasis settings can be determined and adjusted (viadifferent signal compensation steps) while the optical transmittersystem 200 is in use.

The SerDes/DAC module 282 can be a hardware module that can be, forexample, manufactured on a stand-alone integrated circuit chip orincluded in the processor of the optical transmitted system 200. TheSerDes/DAC module 282 can include one or multipleserializer-deserializer/digital-analog converters (SerDes/DACs) that areassociated with the in-phase portion (I-channel) and quadrature portion(Q-channel) of the optical transmitter system 200. The SerDes/DACs canprovide spectrally efficient, multi-level signal generation and spectralmanipulation. Each of the SerDes/DACs (of the SerDes/DAC module 282) caninclude one or both of a serializer/deserializer (SerDes) and adigital-analog converter (DAC). A SerDes (or serializer/deserializer)typically includes a pair of functional blocks commonly used in highspeed communications to compensate for limited input/output. The SerDescan be an integrated circuit transceiver that converts parallel data toserial data and vice-versa. The transmitter section of the SerDes is aparallel-to-serial converter, and the receiver section of the SerDes isa serial-to-parallel converter. Multiple SerDes interfaces can often behoused in a single integrated circuit package. The SerDes facilitatesthe transmission of parallel data between two locations over serialstreams, thus reducing the number of data paths and the number ofconnecting pins or wires used on a device. In some instances, the SerDesdevices (or modules) are capable of full-duplex operation, where dataconversion can take place in both directions simultaneously.

The digital-analog converter (DAC) of each individual SerDes/DAC (of theSerDes/DAC module 282) can convert the digital bit streams of thecalibration signal and/or the data signal (of the real data) into acontinuously varying analog signal. In some instances, thedigital-to-analog conversion can degrade a signal, and hence conversionset-points can be set such that any errors induced in the conversionprocess are minimized. Note that the DACs are optional or not presentfor some modulation formats (e.g., quadrature phase-shift keying (QPSK)modulation also referred to as 4-quadrature amplitude modulation(4-QAM)), as simple on-off-keying (OOK) signals on each channel aresufficient. For such modulation formats, the DACs can be 1-bit DACs.

The pre-emphasis generation module 284 can be hardware module and/orsoftware module stored in the memory and/or executed in a processor ofthe optical transmitter system 200. The pre-emphasis generation module284 can receive a correction control signal from the control module 260that is representative of the calculated parameters of thepre-emphasized signal. The pre-emphasis generation module 284 cangenerate a pre-emphasized signal that can be sent to the SerDes/DACmodule 282 via an out-of-band communication channel. The pre-emphasizedsignal can be sent to the optical transmitter module and can equalize(or compensate) for distortions associated with the first calibrationsignal (at a first frequency range) and the second calibration signal(at a second frequency range) and/or equalize (or compensate) fordistortions associated with the first filtered signal (at a firstfrequency range) and the second filtered signal (at a second frequencyrange). The equalization process can be repeated until the appropriatetone ratio (ratio of the power of the two calibration signals and/orfiltered signals) reaches a pre-determined level that is indicative ofoptimal system performance.

The pre-emphasis generation module 284 can be implemented in someinstances, via digital filtering methods and in other instances, viaanalog filtering methods. Digital filtering can be implemented either inthe time domain with finite-impulse response (FIR) filters or in thefrequency domain with frequency-domain-equalization (FDE) filters. Interms of the filter structure, a digital Finite Impulse Response (FIR)filter in the time-domain can be represented as cascaded tapped delayelements, with each delay element being equal to the symbol period ofthe signal, T, or a fraction of T (e.g., with T/2) and controllableweights (i.e., filter coefficients). Although T/2-spaced FIR filtersoccur typically at the Rx-side (receiver-side) due to sampling at twotimes the symbol rate (2/T), fractionally-spaced FIR filters forpre-emphasis are also currently being developed and/or implemented. Forexample, a time-domain digital FIR filter can be implemented with aweighted sum of tapped delay elements. One method of setting the taps ofthe time-domain digital FIR filter is to derive the desired frequencydomain peaking responses and perform an inverse transform to obtain thetime domain impulse responses (that corresponds to the tap or filtercoefficients). The desired frequency-domain peaking response can beobtained by subtracting the measured channel response (by measuring thepeak detector output when no pre-emphasis filter is set) from thedesired (e.g., flat passband) response. Another method of setting thetaps of the time-domain digital FIR filter is to implement an adaptivealgorithm to minimize the error signal by automatically adjusting thetap coefficients until the error signal falls within a pre-determinedrange.

A digital filter (e.g., FDE) in the frequency-domain can be implementedwith any one of the many Fast Fourier Transform or Inverse Fast FourierTransform (FFT/IFFT) structures (or modules). For example, afrequency-domain digital filter can be implemented using an overlap-savetechnique. With an overlap-and-save FDE filter, the digital inputs aredivided into blocks, such that the block length equals the size of theunderlying fast Fourier transform (FFT) in addition to a number ofoverlap samples between neighboring blocks. Each block is firstconverted into the frequency domain, and after the equalization in thefrequency domain, the block is converted back into the time domain byinverse fast Fourier transform. The overlap in a block can be discardedwhen this block is stored as filtered results.

In some instances, when the pre-emphasis generation module 284 isimplemented via analog filtering methods, the analog filter wouldtypically be located after the DAC (in the SerDes/DAC module 282).Analog filters in the pre-emphasis generation module 284 can beimplemented by several methods, including active and passive designs.For example, in some configurations, the analog filters can beimplemented with a Continuous Time Linear Equalizer (CTLE) approach thatincludes high-pass filtering (HPF) with one or multiple poles and zerosto provide emphasis in the high frequency response. The HPF CTLE analogfilter implementation can also include the use of high frequency polesto force the attenuation of high frequency noise so that SerDes systemperformance is not degraded. In other configurations, the analog filterscan be implemented with a CTLE approach using a feed-forward equalizer(FFE) that can provide emphasis in the high frequency response usingdelays, gains and a summer. The FFE CTLE analog filter implementation isa FIR filter. The input digital signal can propagate through a series ofdelay lines, where each delay line is equal to one bit unit timeinterval (UI). The input signal is sampled before and after each delayline and is multiplied by the FIR tap coefficients (filtercoefficients). The outputs from the FIR taps can be summed to producethe FFE CTLE output. The number of taps can depend on the length of thechannel impulse response relative to one bit unit time interval.

Analog filters can be used as the sole device in the pre-emphasisgeneration module 284 to generate the pre-emphasized signal or can beused in conjunction with a digital filter. If used in conjunction with adigital filter, the analog filter can be implemented after the DAC. Notethat the analog filter alone might not be able to meet all thecompensation targets for the pre-emphasized signal for all applications,but can serve to substantially meet a majority of the compensationtargets, thus leaving the digital filters to compensate for theremaining targets. Note that the components of theserializer/deserializer module 280 shown in FIG. 2 are represented in aspecific order in FIG. 2 by way of example only, and not by limitation.In some configurations, when an analog implementation is used in thepre-emphasis generation module 284, the pre-emphasis generation module284 is located after the SerDes/DAC module 282. In other configurations,when a digital implementation is used in the pre-emphasis generationmodule 284, the pre-emphasis generation module 284 is located before theSerDes/DAC module 282.

FIG. 3 is system block diagram of a control module 300, according to anembodiment. The control module 300 in FIG. 3 is similar to orsubstantially similar to the control module 120 in FIG. 1 and thecontrol module 260 in FIG. 2. As described above, the control module 300can be external to the optical transmitter system or located on the hostcircuit board of the optical transmitter system. When located on thehost circuit board of the optical transmitter system, the control module300 can be a hardware module and/or software module stored in the memoryand/or executed in a processor on the host circuit board of the opticaltransmitter system. The control module 300 includes a memory 340 that isoperably coupled to a processor 320. The control module 300 alsoincludes a communication interface 360 that is operably coupled to boththe memory 340 and the processor 320.

The communications interface 360 of the control module 300 can include,for example, one or multiple input/output ports (not shown in FIG. 3)that can be used to implement one or more connections between thecontrol module 300 and the optical transmitter module and theserializer/deserializer module (e.g., optical transmitter module 210 inFIG. 2 and serializer/deserializer module 280 in FIG. 2). As such, thecontrol module 300 can be configured to receive data signals and/or senddata signals through one or more ports of the communications interface360, which are connected to the communications interfaces of the opticaltransmitter module and the serializer/deserializer module (not shown inFIGS. 2 and 3). In some instances, for example, when the control module300 is external to the optical transmitter system, the control module300 can implement a first wired connection (e.g., current mode logic(CML) signaling, also known as source coupled logic (SCL) signaling,analog signaling, combination of digital and analog signaling, and/orthe like) with the optical transmitter module that can be operativelycoupled to the control module 300 through one port of the communicationinterface 360. In such instances, the control module 300 can implement asecond wired connection (e.g., current mode logic (CML) signaling, alsoknown as source coupled logic (SCL) signaling, analog signaling,combination of digital and analog signaling, and/or the like) with theserializer/deserializer module that is operatively coupled to thecontrol module 300 through another port of the communication interface360 (and vice-versa).

The processor 320 of the control module 300 can be, for example, ageneral purpose processor, a Field Programmable Gate Array (FPGA), anApplication Specific Integrated Circuit (ASIC), a Digital SignalProcessor (DSP), and/or the like. The processor 320 is configured to runand/or execute processes and/or other modules, instructions, and/orfunctions associated with the control module 300. The processor 320includes a calibration signal computation module 322 and a pre-emphasiscalculation module 324. As described above, the control module 300 canreceive the output of the detectors (the first power error signal andthe second power error signal) in the optical transmitter module via oneor multiple input ports of the communication interface 360. Thepre-emphasis calculation module 324 can receive the first power errorsignal and the second power error signal from the communicationinterface 360. As described above, the first power error signal and thesecond power error signal can either be based on an in-band input signal(generated by the serializer/deserializer module) or a first calibrationsignal and a second calibration signal (generated by theserializer/deserializer module).

The pre-emphasis calculation module 324 can compute the parameters of apre-emphasis function based on the properties of the first power errorsignal and the second power error signal (e.g., magnitude of thesignals, phase of the signals, frequency of the signals, etc.). Thepre-emphasis calculation module 324 can generate a correction controlsignal that contains or represents information and/or instructions aboutthe pre-emphasis function and can send the correction control signal tothe electrical serializer/deserializer module. Theserializer/deserializer module can generate and/or modify thepre-emphasized signal and send the pre-emphasized signal to the opticaltransmitter module such that the frequency-dependent distortions of theoptical transmitter system can be substantially compensated (orequalized). In some instances, the equalization process described abovecan be a recursive process. In such instances, multiple pre-emphasizedsignals can be generated and sent to the optical transmitter moduleuntil the ratio of the power of the first power error signal and thepower of the second power error signal is substantially equal to orbelow a pre-determined value (the pre-emphasis loop or circuit isconverged). In some configurations, the pre-emphasis calculation module324 can also generate and send a signal warning of an impending powerfailure of a SerDes (or DAC) module before regulated DC voltages in thehost circuit board of the optical transmitter system (e.g., of theoptical transmitter module or the serializer/deserializer module) goesout of the established specification range by monitoring the power (orvoltage) levels in the first power error signal and/or the second powererror signal. This can facilitate a pre-emptive replacement orswitchover to another datapath, or a timely and orderly shutdown and anautomatic restart of the optical transmitter system.

The calibration pattern computation module 322 can run and/or executeprocesses and/or other modules, instructions, and/or functionsassociated with the generation of the calibration digital bit streams atdifferent frequency ranges. The calibration pattern computation module322 can send (via the communication interface 360) to the calibrationpattern generation module (e.g., calibration pattern generation module286 in FIG. 2) a signal that contains or represents information and/orinstructions about the calibration signal to send to the I-channel andthe Q-channel of the optical transmitter system. For example, in someinstances, the calibration signal computation module 322 can sendinstructions to the calibration signal generation module to generate andsend a “11001100” repeating bit stream that corresponds to 8 GHz for a32 Gbps data stream as the first calibration signal, and a“11111111000000001111111100000000” repeating bit stream that correspondsto 2 GHz for a 32 Gbps data stream as the second calibration signal. Theparameters of the calibration signal (e.g., calibration bit streamprofile, frequency of the calibration bit stream, etc.) can be computedand set by the calibration signal computation module 322 based on thecompensation desired, for example, to minimize distortion caused byfrequency dependent loss in the connection between the SerDes/DAC module(e.g., SerDes/DAC module 282 in FIG. 2) and the amplifiers (e.g.,amplifiers 222 and 224 in FIG. 2).

The memory 340 can be, for example, a random access memory (RAM), amemory buffer, a hard drive, a database, an erasable programmableread-only memory (EPROM), an electrically erasable read-only memory(EEPROM), a read-only memory (ROM), a flash memory, and/or so forth. Thememory 340 can store data and instructions to cause the processor 320 toexecute modules, processes and/or functions associated with the controlmodule 300. The memory 340 includes a power database 342 and apre-emphasis calculation database 344. In some instances, the powerdatabase 342 can store the values of the power associated with the firstpower error signal and the second power error signal for a single stepequalization process. In others instances, the power database 342 canstore the values of the power associated with the first power errorsignal and the second power error signal for each individual step of amultiple-step recursive equalization process. The power database 342 canalso store the pre-determined tone ratio value (the desired value of theratio of the power of the first power error signal to that of the powerof the second power error signal) that is indicative of optimalperformance of the optical transmitter system. The desired tone ratiovalue can be, for example, a value that can be manually set by a user ora manufacturer of the optical transmitter system.

The pre-emphasis calculation database 344 can store instructions and/orinformation that can be accessed by the pre-emphasis calculation module324 to calculate the parameters of the pre-emphasized signal that canallow for optimal equalization of the transmitted signal from theoptical transmitter system based on the output value of the detectors(of the optical transmitter module), (in some instances) the digital bitstream of the calibration signals, and frequency of the calibrationsignals. The entries of the pre-emphasis calculation database 344 can beupdated repeatedly, on a periodic interval or on a substantiallyperiodic interval. The pre-emphasis calculation database 344 can containor represent the current information and/or instructions that can beused by the pre-emphasis calculation module 344 to compute thepre-emphasis parameters suitable for the optical transmitter systemduring the device startup, after reset, or can be continuously optimizedin the background to compensate for changes in distortion ortime-varying distortions (that can result in frequency-basedfluctuations in the power of the transmitted signal). In someconfigurations, the pre-emphasis calculation database 344 can also storeinstructions and/or information that can be accessed by the pre-emphasiscalculation module 324 to generate the correction control signal.

FIG. 4 is a graphical example of an uncompensated signal (or channel), apre-emphasis (transfer or adjustment) function and the resultingcompensated (or equalized) signal (or channel). The abscissa of thegraph represents the frequency of operation of the optical transmittersystem in arbitrary units (a.u.), and the ordinate of the graphrepresents the power of the transmitted signal in arbitrary units(a.u.). The uncompensated (transmitter output) signal displaysfrequency-based fluctuations in power. The power of the uncompensatedsignal has been shown to be monotonically decreasing with increasingfrequency in FIG. 4 by way of example only and not by limitation. Inother instances, the power of the uncompensated signal can displayfrequency-based fluctuations that can have other periodic,semi-periodic, random and/or non-random profiles. As described above, insome instances, the pre-emphasis function can be applied at theserializer/deserializer module (e.g., serializer/deserializer module 280in FIG. 2) to compensate for the frequency-based fluctuations in thepower (e.g., due to changes in distortion and/or time-varyingdistortions) of the transmitted signal (equalization process). Hence,the pre-emphasis function can counteract or alleviate thefrequency-based power (or intensity) fluctuations of the uncompensatedsignal presented in FIG. 4. The compensation or equalization processleads to the generation of the equalized or compensated signal (shown bythe bold line in FIG. 4) that can display a uniform or substantiallyuniform power (or intensity) levels across a wide frequency range. Theprofile of the pre-emphasis function has been shown to be monotonicallyincreasing with increasing frequency in FIG. 4 by way of example only,and not by limitation. The pre-emphasis function shown in FIG. 4 is anexample of a transfer function that can compensate for thefrequency-based power fluctuations of the specific uncompensated signalshown in FIG. 4. In other instances, the frequency-based profile of thepre-emphasis function can have any periodic, semi-periodic, randomand/or non-random profiles that can substantially compensate for thefrequency-based power fluctuation for a variety of uncompensatedsignals. Note that the power (or intensity) levels of the compensatedsignal has been shown to be substantially uniform or flat over thedesired frequency range in FIG. 4 by way of example only, and not bylimitation. In other instances, the power (or intensity) levels of thecompensated signal can also vary within a desired frequency range.

FIG. 5 is a graphical example of time domain eye diagrams for: (a) anuncompensated broadband on-off-keyed signal; and (b) a compensatedbroadband on-off-keyed signal. The eye diagram is an oscilloscopedisplay in which a digital data signal from an electrical device isrepetitively sampled and applied to the vertical input or axis, whilethe data rate is used to trigger the horizontal axis sweep. Because highspeed digital signals can exceed multiple Gigabit per second (Gbps)speeds, eye diagrams can provide a way to measure signal quality andsystem performance. The eye diagram allows different parameters of theelectrical quality of the signal to be (quickly) visualized anddetermined. The eye diagram is constructed from a digital waveform byfolding the parts of the waveform corresponding to each individual bitinto a single graph with signal amplitude on the vertical axis and timeon the horizontal axis. FIG. 5(a) is a graphical example of a timedomain eye diagram for an uncompensated broadband on-off-keyed (OOK)signal. Eye height is a measure of the vertical opening of an eyediagram as shown by the dotted line marked “A” in FIG. 5(b). The eyeheight measurement is an indication of the eye closure due to noise anddistortion. The signal to noise ratio of a high speed data signal isalso directly indicated by the amount of eye closure. The eye diagram ofthe uncompensated signal in FIG. 5(a) shows a relatively low value forthe eye height (i.e., displaying eye closure). The low value for eyeheight is indicative of the presence of noise in the signal or a lowsignal to noise ratio. An “ideal” eye opening measurement (i.e., withlow noise or substantially low noise) would be equal to the eyeamplitude measurement, where the eye amplitude is the difference betweenthe one and zero levels of the eye diagram as shown by the dotted linemarked “B” in FIG. 5(b). The eye diagram of the uncompensated signal inFIG. 5(a) also does not show a clear eye pattern during each unitinterval (UI) width of the digital bit pattern, and the rise time andfall time of the eye pattern cannot be computed in the time windowpresented in FIG. 5(a). This can be indicative of significant signalimpairment due to, for example, distortion, attenuation, noise,crosstalk, jitter, and/or the like.

Compensation or equalization of the uncompensated signal usingpre-emphasis (e.g., the pre-emphasis function as described in FIGS. 1-4)leads to the generation of the compensated signal. The time domain eyediagram of the compensated broadband OOK signal is shown in FIG. 5(b).The eye diagram of the compensated signal in FIG. 5(b) shows asubstantial increase in the eye height, the presence of clear eyepatterns during each unit interval (UI) width of the digital bitpattern, and a shorter rise time and fall time (approximately 20 ps)that can be calculated in the time window presented in FIG. 5(b). Suchsignificant improvement in the eye diagram of the compensated signal isindicative of the utility of using the pre-emphasis to compensate forthe frequency-based power fluctuations of the transmitted signal fromthe optical transmitter system. Note that an OOK signal is presented inFIG. 5 as an example of a possible signal that can be compensated orequalized by the method described in FIGS. 1-4. OOK signals areapplicable for digital modulation schemes such as, for example, binaryphase-shift keying (BPSK) modulation and quadrature phase-shift keying(QPSK) modulation. In other instances, other type of signals (e.g.,multi-level signals) can be compensated or equalized by the methoddescribed in FIGS. 1-4 for other modulation techniques such as, forexample, quadrature amplitude modulation (QAM) that is both an analogand a digital modulation scheme.

FIG. 6 is a flow chart illustrating a method for the compensation offrequency-based transmitted signal power fluctuations, according to afirst embodiment. The method 400 includes sending an input in-bandsignal from, for example, a serializer/deserializer module to, forexample, an optical transmitter module, at 402. As described above, insome instances, the serializer/deserializer module can be a hardwaremodule and can generate the input in-band signal and can send thein-band signal to the I-channel modulator and the Q-channel modulator ofthe optical transmitter system via a set of amplifiers (or drivers). Asdescribed above, the optical transmitter module can include one ormultiple electrical detectors that are operably coupled to theamplifiers. The electrical detectors can sample a portion of the in-bandsignal. In some instances, the detectors can include one or multiplefilters that can filter the in-band signal at different frequencyranges.

The input in-band signal is filtered at a first frequency range toproduce a first filtered signal and at a second frequency range toproduce a second filtered signal, at 404. As described above, the firstfrequency range is different from the second frequency range. In suchinstances, the amplifiers (or drivers) in the optical transmitter moduleare activated (i.e., not disabled) when the detectors in the opticaltransmitter module receive the in-band input signal from theserializer/deserializer module. In such instances, the optical shutterdoes not have to be in the closed configuration to block the transmittedsignal of the optical transmitter system. In some configurations, theelectrical detectors may not include separate filter units or modules,but the frequency range of operation of the electrical detectors canserve as a de-facto filtration mechanism for the in-band signal togenerate the first filtered signal and the second filtered signal.

The first power error signal and the second power error signal aregenerated at, for example, the optical transmitter module, at 406. Asdescribed above, the first power error signal is based on the firstfiltered signal and the second power error signal is based on the secondfiltered signal. The power error signals are generated by thedetector(s) of the optical transmitter module and can be based on theproperties of the filtered signals (i.e., magnitude of the signals,phase of the signals, frequency of the signals, etc.). After generationof the first power error signal and the second power error signal, thefirst power error signal and the second power error signal can be sentfrom, for example, the optical transmitter module to, for example, thecontrol module, at 408.

The first power error signal and the second power error signal can beprocessed and compared at, for example, the control module to determineif there are notable frequency-based signal power (or intensity)fluctuations, at 410. The first power error signal and the second powererror signal can be received by the control module via one or multipleports of the communication interface of the control module. As describedabove, the processing and analysis of the power error signals can beperformed by, for example, comparing the magnitude of the first powererror signal to the magnitude of the second power error signal anddetermining if the ratio of the magnitudes (or tone ratio of the powererror signal) is less than a pre-determined level. Said in another way,the comparison of the magnitude of the first power error signal to themagnitude of the second power error signal is performed to determine ifthe difference in power between the first error signal and the seconderror signals is within an acceptable (pre-determined) range. Suchcomputations can take place, for example, in the processor of thecontrol module. In some instances, the ratio of the magnitude of thepower error signals can be less than or equal to the pre-determinedlevel. In such instances, the (low) value of the ratio the power errorsignals can indicate that notable frequency-based signal power (orintensity) fluctuations do not exist (i.e., the difference in powerbetween the first error signal and the second error signals can bewithin an acceptable range) and the method for the compensation offrequency-based transmitted signal power fluctuations ends. Note that inthis embodiment, the input signal that is being filtered (at twofrequency ranges) to generate the first power error signal and thesecond power error signal is the in-band input data (i.e., actual data).In such embodiments, the power spectrum of the input signal is not knownby the SerDes/DAC module before the input signal is sent to the opticaltransmitter module. Hence, a portion of the in-band input signal can betapped off and used by, for example, the SerDes/DAC module to measurethe power spectrum of the input signal before the input signal is sentto the optical transmitter module. Such measurements can serve as thereference signals, which are compared against the power spectrum of thesignals received by the detectors (e.g., detectors 230 and 234 in FIG.2), to determine the frequency-based distortion introduced by theoptical transmitter system. In this situation, although not shown inFIG. 2 or 6, serializer/deserializer module 280 can send referencesignals to control module 260 so that control module 260 can perform thecomparison of the reference signals against the power spectrum of thesignals received by the detectors.

In other instances, the ratio of the magnitude of the power errorsignals can be greater than the pre-determined level (i.e., thedifference in power between the first error signal and the second errorsignals is not within an acceptable pre-determined range). In suchinstances, the value of the ratio of the power error signals canindicate that notable frequency-based signal power (or intensity)fluctuations may exist. In such instances, a correction control signalis generated at, for example, the control module that is based on thefirst power error signal and the second power error signal, at 412. Asdescribed above, the correction control signal can be generated based onthe properties of the power error signals (e.g., the relative magnitudeof the power error signals, the phase of the power error signals, thefrequency of the power error signals, etc.). After generation of thecorrection control signal, the correction control signal can be sentfrom, for example, the control module to, for example, theserializer/deserializer module via the a port on the communicationinterface of the control module.

After successful reception of the correction control signal at, forexample, the serializer/deserializer module, the parameters of thepre-emphasis function can be modified at the serializer/deserializermodule based on the correction control signal, at 414. The pre-emphasisfunction can include information that can be used in the equalization orcompensation of the uncompensated signal at the two frequency rangesused to generate the first power error signal and the second power errorsignal. For frequencies outside the above mentioned frequency ranges,linear or non-liner interpolation methods can be applied by thepre-emphasis generation module to generate the pre-emphasized signal.

The pre-emphasis function can be applied to the uncompensated signal, atfor example, the serializer/deserializer module to generate thepre-emphasized signal, at 416. As described above, the pre-emphasizedsignal can counteract or alleviate the frequency-based powerfluctuations introduced by the channel. This compensation orequalization process can lead to the generation of the equalized (orcompensated) channel that can display a uniform or substantially uniformsignal power levels across a wide frequency range (or across the entireoperational frequency range of the optical transmitter system). Thefrequency-based power profile of the pre-emphasized signal can be forexample, monotonically increasing with increasing frequencies,monotonically decreasing with increasing frequencies, or can have anyperiodic, semi-periodic, random and/or non-random profiles that cancompensate for the frequency-based power profile of a variety ofchannels. The signal equalization (or compensation) process iteratesuntil the desired pre-emphasis correction applied to the signal is lessthan a pre-determined value, at which point the signal compensation loopis considered to have converged. The (final) pre-emphasized signal issent from, for example, the serializer/deserializer module to, forexample, the optical transmitter module, at 418.

FIG. 7 is a flow chart illustrating a method for the compensation offrequency-based transmitted signal power fluctuations, according to asecond embodiment. The method 500 includes, optionally (as denoted bythe dashed box), closing the optical shutter (e.g., located after theI-channel and Q-channel modulators in FIG. 2) before the tunable laseris enabled and the calibration signals are generated so that the opticaltransmitter system output can be blocked during the during signalequalization (or compensation) process, at 502.

A first calibration signal and a second calibration signal can begenerated at, for example, the serializer/deserializer module, at 504.The first calibration signal includes digital data associated with afirst frequency and the second calibration signal includes digital dataassociated with a second frequency, where the first frequency isdifferent from the second frequency. As described above, the (digital)calibration signals can be a specific kind of electrical waveformvarying between two voltage levels that correspond to two logic states(e.g., low state′ for ‘0’ and ‘high state’ for ‘1’, respectively). Thevoltage levels generated by the calibration signal generation module canbe compatible with digital electronics input/output (I/O) standards suchas, for example, Transistor-Transistor Logic (TTL), Low-voltage TTL(LVTTL), Low Voltage Complementary Metal Oxide Semiconductor (LVCMOS),Low-voltage differential signaling (LVDS), and/or the like. As describedabove, in some instances, the first calibration signal can be a“11001100” repeating digital bit stream that corresponds to 8 GHz for a32 Gbps data stream, and the second calibration signal can be a“11111111000000001111111100000000” repeating digital bit stream thatcorresponds to 2 GHz for a 32 Gbps data stream. The parameters of thecalibration signal (e.g., calibration digital bit stream profile,frequency of the calibration digital bit stream, etc.) can be computedand set by the calibration signal computation module (of the controlmodule) based on the compensation desired for the frequency-basedtransmitted signal power fluctuations. The first calibration signal andthe second calibration signal are sent from, for example, theserializer/deserializer module, to, for example, the optical transmittermodule, at 506.

The first power error signal and the second power error signal aregenerated at, for example, the optical transmitter module, at 508. Asdescribed above, the first power error signal is based on the firstcalibration signal and the second power error signal is based on thesecond calibration signal. The power error signals are generated by thedetector(s) of the optical transmitter module and can be based on theproperties of the calibration signals (e.g., magnitude of the signals,phase of the signals, frequency of the signals, etc.). After generationof the first power error signal and the second power error signal, thefirst power error signal and the second power error signal are sentfrom, for example, the optical transmitter module to, for example, thecontrol module, at 510.

The first power error signal and the second power error signal can beprocessed and compared at, for example, the control module to determineif there are notable frequency-based signal power (or intensity)fluctuations, at 512. As described above, the processing of the powererror signals can be performed by, for example, comparing the magnitudeof the first power error signal to the magnitude of the second powererror signal and determining if the ratio of the magnitudes (or toneratio of the power error signal) is less than a pre-determined level.Said in another way, the comparison of the magnitude of the first powererror signal to the magnitude of the second power error signal isperformed to determine if the difference in power between the firsterror signal and the second error signals is within an acceptable(pre-determined) range. As described above, such processing can takeplace, for example, in the processor of the control module. In someinstances, the ratio of the magnitude of the power error signals can beless than or equal to the pre-determined level. In such instances, thevalue of the ratio the magnitude of the power error signals can indicatethat notable frequency-based signal power (or intensity) fluctuations donot exist (i.e., the difference in power between the first error signaland the second error signals is within an acceptable range). In suchinstances, the transmission of data can be enabled and optionally (asindicated by the dashed box), the optical shutter can be brought intothe second configuration (i.e., opened) and the method for thecompensation of the frequency-based transmitted signal powerfluctuations can end, at 514.

In other instances, the ratio of the magnitude of the power errorsignals can be greater than the pre-determined level (i.e., thedifference in power between the first error signal and the second errorsignals is not within an acceptable pre-determined range). In suchinstances, the value of the ratio of the power error signals canindicate that notable frequency-based signal power (or intensity)fluctuations may exist. In such instances, a correction control signalis generated at, for example, the control module that is based on thefirst power error signal and the second power error signal, at 516. Thecorrection control signal can be generated based on the properties ofthe power error signals as described above. After generation of thecorrection control signal, the correction control signal can be sentfrom, for example, the control module to, for example, theserializer/deserializer module as described above.

After successful reception of the correction control signal at, forexample, the serializer/deserializer module, the parameters of thepre-emphasis function can be modified at, for example, theserializer/deserializer module based on the correction control signal,at 518. The pre-emphasis function can include information that can beused in the equalization or compensation of the uncompensated signal atthe frequency ranges of the calibration signals used to generate thefirst power error signal and the second power error signal. Forfrequencies outside the above mentioned frequency ranges, linear ornon-liner interpolation methods can be applied by the pre-emphasisgeneration module to generate the pre-emphasized signal.

The pre-emphasis function can be applied to the uncompensated signal, atfor example, the serializer/deserializer module to generate thepre-emphasized signal, at 520. As described above, the pre-emphasizedsignal can counteract or alleviate the frequency-based powerfluctuations of the channel. This compensation or equalization processcan lead to the generation of the equalized (or compensated) channelthat can display a uniform or substantially uniform signal power levelsacross a wide frequency range (or across the entire operationalfrequency range of the optical transmitter system). The frequency-basedpower profile of the pre-emphasized signal can be for example,monotonically increasing with increasing frequencies, monotonicallydecreasing with increasing frequencies, or can have any periodic,semi-periodic, random and/or non-random profiles that can compensate forthe frequency-based power profile of a variety of uncompensated signals.The signal equalization (or compensation) process iterates until thedesired pre-emphasis correction applied to the signal is less than apre-determined value, at which point the signal compensation loop isconsidered to have converged. The (final) pre-emphasized signal is sentfrom, for example, the serializer/deserializer module to, for example,the optical transmitter module, at 522.

The methods and apparatus shown in FIGS. 1-7 can automatically set theSerDes TX pre-emphasis settings for an optical transmitter system thatcan compensate for fluctuations in the power levels of transmittedsignal due to, for example, the effects of temperature variations andtemporal variations. Known methods can be suitable for 10G systems suchas those that use small form-factor pluggable (SFP)+ interfaces, but arenot adequate for high symbol rate and high performance applications suchas 100G coherent Dense Wave Division Multiplexing (DWDM) interfaces (25Gigasymbol/s to 32 Gigasymbol/s). The demands for signal compensationgrow more stringent as the symbol rate (Baud rate) increases and/or theorder of modulation increases (i.e., encoding more bits per symbol) suchas those for 16QAM at 4 bits/Symbol (compared to 4QAM at 2bits/symbol).

The methods and apparatus shown in FIGS. 1-7 have the advantage ofproviding optimization of the pre-emphasized signal through acombination of an out-of-band communication channel between the opticaltransmitter module (a pluggable module) and the SerDes/DAC module,electrical detector(s) in the optical transmitter module, andpre-determined digital calibration signals (e.g., digital bit streams atdifferent frequencies) that are sent to the SerDes/DAC module.Additionally, the detectors can also be small, low power circuits thatcan be readily added to the amplifiers in a variety of photonic modules.The methods and apparatus shown in FIGS. 1-7 can also be applied to orembodied in a variety of optical transmitter systems such as, forexample, an optical M-ary quadrature amplitude modulated (M-QAM)transmitter, a polarization multiplexed (PM) M-QAM transmitter where twoor more M-QAM transmitters in substantially orthogonal polarizationstates are multiplexed in the polarization domain. The M-QAM transmittercan be paired with either a direct detection receiver or a coherentreceiver. Specific examples include direct detection differential QPSK(DOPSK), coherent PM-QPSK (i.e., coherent PM-4-QAM), coherent PM-16-QAM,and/or the like. Additionally, the methods and apparatus describedherein can apply to both a single polarization transmitter and apolarization-multiplexed transmitter.

Some embodiments described herein relate to a computer storage productwith a non-transitory computer-readable medium (also can be referred toas a non-transitory processor-readable medium) having instructions orcomputer code thereon for performing various computer-implementedoperations. The computer-readable medium (or processor-readable medium)is non-transitory in the sense that it does not include transitorypropagating signals per se (e.g., a propagating electromagnetic wavecarrying information on a transmission medium such as space or a cable).The media and computer code (also can be referred to as code) may bethose designed and constructed for the specific purpose or purposes.Examples of non-transitory computer-readable media include, but are notlimited to: magnetic storage media such as hard disks, floppy disks, andmagnetic tape; optical storage media such as Compact Disc/Digital VideoDiscs (CD/DVDs), Compact Disc-Read Only Memories (CD-ROMs), andholographic devices; magneto-optical storage media such as opticaldisks; carrier wave signal processing modules; and hardware devices thatare specially configured to store and execute program code, such asApplication-Specific Integrated Circuits (ASICs), Programmable LogicDevices (PLDs), Read-Only Memory (ROM) and Random-Access Memory (RAM)devices. Other embodiments described herein relate to a computer programproduct, which can include, for example, the instructions and/orcomputer code discussed herein.

Examples of computer code include, but are not limited to, micro-code ormicro-instructions, machine instructions, such as produced by acompiler, code used to produce a web service, and files containinghigher-level instructions that are executed by a computer using aninterpreter. For example, embodiments may be implemented usingimperative programming languages (e.g., C, Fortran, etc.), functionalprogramming languages (Haskell, Erlang, etc.), logical programminglanguages (e.g., Prolog), object-oriented programming languages (e.g.,Java, C++, etc.) or other suitable programming languages and/ordevelopment tools. Additional examples of computer code include, but arenot limited to, control signals, encrypted code, and compressed code.

While various embodiments have been described above, it should beunderstood that they have been presented by way of example only, and notlimitation. Where methods described above indicate certain eventsoccurring in certain order, the ordering of certain events may bemodified. Additionally, certain of the events may be performedconcurrently in a parallel process when possible, as well as performedsequentially as described above.

What is claimed is:
 1. An apparatus, comprising: an optical transmittermodule configured to be electrically coupled to an electricalserializer/deserializer and a controller, the optical transmitter moduleincluding an electrical detector configured to receive an in-bandsignal, the electrical detector configured to send to a controller afirst power error signal based on the in-band signal such that thecontroller sends a correction control signal to the electricalserializer/deserializer based on the first power error signal and asecond power error signal and such that the electricalserializer/deserializer modifies a pre-emphasized signal to the opticaltransmitter module based on the correction control signal, the firstpower error signal, the second power error signal and the correctioncontrol signal being out-of-band.
 2. The apparatus of claim 1, wherein:the electrical detector is a first electrical detector, the firstelectrical detector is configured to filter the in-band signal for afirst frequency range to produce a first filtered signal, a secondelectrical detector configured to filter the in-band signal for a secondfrequency range different from the first frequency range to produce asecond filtered signal, the first power error signal associated with thefirst filtered signal, the second power error signal associated with thesecond filtered signal.
 3. The apparatus of claim 1, wherein: theelectrical detector is a first electrical detector, the in-band signalis a first calibration signal having a predetermined pattern associatedwith a first frequency, a second electrical detector configured toreceive a second calibration signal that is in-band, the secondcalibration signal having a predetermined pattern associated with asecond frequency different from the predetermined pattern associatedwith the first frequency, the first power error signal is associatedwith the first calibration signal, the second power error signal isassociated with the second calibration signal.
 4. The apparatus of claim3, wherein the optical transmitter module includes an optical shutterand an output, the optical shutter configured to block an output signalof the optical transmitter module when the first electrical detector andthe second electrical detector receives the first calibration signal andthe second calibration signal.
 5. The apparatus of claim 3, wherein: theoptical transmitter module includes an I-phase modulator and a Q-phasemodulator, the I-phase modulator is coupled to the first electricaldetector, the Q-phase modulator is coupled to the second electricaldetector, the second electrical detector is configured to receive thefirst calibration signal in addition to the second calibration signal,the first electrical detector and the second electrical detector arecollectively configured to send to the controller the first power errorsignal and the second power error signal.
 6. The apparatus of claim 1,wherein the optical transmitter module is a pluggable module.
 7. Anapparatus, comprising: a controller configured to be operatively coupledto an electrical serializer/deserializer and an optical transmittermodule having an electrical detector, the controller configured toreceive, from the electrical detector, a first power error signal basedon an in-hand signal, the controller configured to send a correctioncontrol signal to the electrical serializer/deserializer based on thefirst power error signal and a second power error signal such that theelectrical serializer/deserializer modifies a pre-emphasis setting basedon the correction control signal, the first power error signal, thesecond power error signal and the correction control signal beingout-of-band.
 8. The apparatus of claim 7, wherein: the electricaldetector is a first electrical detector, the first power error signal isassociated with a first filtered signal produced by the first electricaldetector by filtering the in-band signal for a first frequency range,the second power error signal is associated with a second filteredsignal produced by a second electrical detector by filtering the in-bandsignal for a second frequency range different from the first frequencyrange, the first power error signal associated with the first filteredsignal, the second power error signal associated with the secondfiltered signal.
 9. The apparatus of claim 7, wherein: the electricaldetector is a first electrical detector, the in-band signal being afirst in-band signal, the controller configured to receive from thefirst electrical detector the first power error signal, and from thesecond electrical detector the second power error signal, based on thefirst in-band signal and a second in-band signal, the first in-bandsignal is a first calibration signal having a predetermined patternassociated with a first frequency, the second in-band signal is thesecond calibration signal having a predetermined pattern associated witha second frequency different from the predetermined pattern associatedwith the first frequency, the first power error signal is associatedwith the first calibration signal, the second power error signal isassociated with the second calibration signal.
 10. The apparatus ofclaim 7, wherein the controller is located within the opticaltransmitter module.
 11. The apparatus of claim 7, wherein the controlleris located within an integrated circuit that includes the electricalserializer/deserializer.
 12. The apparatus of claim 7, wherein thecontroller is physically separate from the optical transmitter moduleand the electrical serializer/deserializer.
 13. An apparatus,comprising: an electrical serializer/deserializer configured to beoperatively coupled to a controller and an optical transmitter modulehaving an electrical detector, the electrical serializer/deserializerconfigured to send to the electrical detector an in-band signal suchthat the optical transmitter module sends to the controller a firstpower error signal, the electrical serializer/deserializer configured toreceive from the controller a correction control signal based on thefirst power error signal and a second power error signal, the electricalserializer/deserializer configured to send to the optical transmittermodule a pre-emphasized signal based on the correction control signal,the first power error signal, the second power error signal and thecorrection control signal being out-of-band.
 14. The apparatus of claim13, wherein: the electrical detector is a first electrical detector, thefirst power error signal is associated with a first filtered signalproduced by the first electrical detector by filtering the in-bandsignal for a first frequency range, the second power error signal isassociated with a second filtered signal produced by a second electricaldetector by filtering the in-band signal for a second frequency rangedifferent from the first frequency range, the first power error signalassociated with the first filtered signal, the second power error signalassociated with the second filtered signal.
 15. The apparatus of claim13, wherein: the electrical detector is a first electrical detector, thein-band signal being a first in-band signal, the controller configuredto receive from the first electrical detector the first power errorsignal, and from a second electrical detector the second power errorsignal, based on the first in-band signal and a second in-band signal,the first in-band signal is a first calibration signal having apredetermined pattern associated with a first frequency, the secondin-band signal is the second calibration signal having a predeterminedpattern associated with a second frequency different from thepredetermined pattern associated with the first frequency, the firstpower error signal is associated with the first calibration signal, thesecond power error signal is associated with the second calibrationsignal.
 16. The apparatus of claim 13, wherein: the in-band signal is afirst in-band signal, the electrical detector is a first electricaldetector, the electrical serializer/deserializer configured to send thefirst in-band signal to the first electrical detector and a secondin-band signal to a second electrical detector of the opticaltransmitter module, such that the optical transmitter module sends tothe controller the first power error signal and the second power errorsignal based collectively on the first in-band signal and the secondin-band signal.
 17. The apparatus of claim 16, wherein: the opticaltransmitter module includes an I-phase modulator and a Q-phasemodulator, the I-phase modulator is coupled to the first electricaldetector, the Q-phase modulator is coupled to the second electricaldetector.
 18. The apparatus of claim 15, wherein: the opticaltransmitter module includes an I-phase modulator and a Q-phasemodulator, the I-phase modulator is coupled to the first electricaldetector, the Q-phase modulator is coupled to a second electricaldetector, the electrical serializer/deserializer is configured to sendto the first calibration signal and the second calibration signal to thesecond electrical detector.
 19. The apparatus of claim 15, wherein: theelectrical serializer/deserializer includes a first digital-to-analogconverter (DAC) and a second DAC, the electrical serializer/deserializerconfigured to send the first calibration signal and the secondcalibration signal from the first DAC to the first electrical detector,the serializer/deserializer configured to send the first calibrationsignal and the second calibration signal from the second DAC to thesecond electrical detector, the optical transmitter module includes anI-phase modulator and a Q-phase modulator, the I-phase modulator iscoupled to the first electrical detector, the Q-phase modulator iscoupled to the second electrical detector.